I've seen both ends of the spectrum -- middle school kids losing their virginity and people still being virgins even into their 20s.
I'd refrain from calling her a whore, though. Sorry, but I just hate that word for reasons unsaid. Attention-seeker, maybe. Low self-esteem, perhaps. Whore, no because it implies so much more on your behalf.
The sex isn't the issue that needs to be fixed, it's the way that she thinks that needs to be fixed. That's the main problem, not the sex. I would just make sure that she's getting some serious education on sexual safety, getting tested, and so on.
Or maybe she's just a rising nymphomaniac and this pattern of behavior will trend into her adult life. I'm close friends with a person at a top school who's just... always been sexual, even when she was young.
It's not usual or even considered "normal" by most people to have so much underage sex, but stop looking at her as a whore and look at the root cause of her behavior because informing her that she's being a whore won't do anything and serves as just a MASSIVE character insult, but informing her that she's being too attention-seeking, or telling her that she doesn't need men to validate her is much more helpful.